The Need for This Post
When I started writing this blog my goal was to present a useful explanation of electricity and the electric industry without using the technical terms and formulas for Watts, Volts, Amps and Ohms that have so long challenged physics students. However, I have been advised that my discussion without some explanation of these terms. So with some reluctance I write this post.
Electrical power is measured in Watts (W) or megawatts (MW). Each MW is equal to 1,000,000 watts.
Since it takes 100 watts to light a 100 watt electrical light bulb, a typical power plant, rated at 500 MW, should produce enough power to light a community consisting of 5,000,000 of these 100 watt light bulbs.
Two things of note here. First, although a 500 MW plant might be built to serve a community consisting of 5,000,000 light bulbs the 5,000,000 light bulbs are not likely to all be in use at the same time. They might all be lit from 7 PM to 10 PM on a typical night. But during other hours of the day fewer than all 5,000,000 will be lit.
The electric utility industry has always dealt with this challenge. It must build facilities required to meet customer usage at the time of the system peak. But during off-peak hours much of the utility plant will be out of use. Utilities always viewed this idle capacity as a wasted opportunity. They wanted to make maximum use of their plant. They hoped to sell enough electricity during off-peak hours to “level out the load curve”. With the support of their regulators they implemented “declining block rate structures” with price discounts that encouraged off-peak consumption.
In the 21stcentury we are more concerned with conservation than with encouraging use of idle capacity. Therefore, utilities no longer implement declining block rate structures to encourage off-peak consumption. Instead, they now seek to level out the load curve by implementing programs to encourage customers to reduce on-peak usage.
The second thing to note about our example of a 500 MW power plant is that the 500 MW plant will not really light 5,000,000 light bulbs. As will be explained in more detail below, a portion of the 500 MW produced at the plant (approximately 5%) will be lost to resistance as it travels on the transmission system. Thus, the 500 MW plant will actually only light 4,750,000 100 watt light bulbs.
Voltage and current
Voltage (measured in volts) is the pressure that pushes electric power through the circuit. Current (measured in amperes or amps) is the speed by which the electric power moves in the circuit.
A typical generating plant produces electricity with between 2,300 volts and 22,000 volts. In order to push the electric power on long distance transmission lines transformers located at the generating plant step up the voltage to between 69,000,000 volts and 765,000,000 volts.
After traveling on the high voltage transmission lines the electricity goes to a local substation where step down transformers convert it to voltages of 35,000 volts or less. These distribution level voltages are then reduced to 110 volts or 220 volts for household use by transformers located in the boxes that we see hanging on utility poles in our neighborhoods.
Power, voltage and current are related by the following formula:
Power (in watts) = Voltage (in volts) x current (in amps)
The takeaway here is that, when the quantity of power is fixed, current can be increased by reducing voltage and current can be decreased by increasing voltage.
Resistance (measured in Ohms) is the degree to which a material or device reduces electric current flowing through it. The copper wire over which electricity flows has resistance that reduces the amount of electrical power available for usage. As indicated above, the resistance in copper wire used in high voltage transmission lines reduces power flowing over it by approximately 5%.
The resistance of any material is inherent in that material. However, the quantity of losses that result from transmission of electricity over that material can be varied.
By combining several complicated formulas it can be seen that losses resulting from resistance on the lines are directly proportional to the current in amps squared. Therefore, line losses can be reduced by reducing the current on the line. And as indicated in the formula in the last section of this Post current can be reduced by increasing voltage. Therefore, the higher the voltage used for transmission, the lower the line losses and the more efficient the electricity delivery. Engineers try to use high voltage lines where possible to reduce the losses of electric power delivered on the system.
This issue of line losses associated with transmission leads to one of the benefits of the use of distributed generation. Because distributed generation is located close to the point of use the electricity that it produces is not subject to the line losses that occur in long distance transmission.
I. David Rosenstein worked as a consulting engineer and attorney in the electric industry for 40 years. At various times during his career he worked for utility customers, Rural Electric Cooperatives, traditional investor owned regulated utilities and deregulated power generation companies. Each of his posts in this blog describes a different aspect of the past, present or future of the electric industry.